1. Field of the Invention
This invention relates to a photometric circuit to be used in a camera or the like, and more particularly to a photometric circuit which logarithmically compresses a result of a light measurement by utilizing the voltage-current characteristic of the p-n junction of semiconductors and integrates the result of the light measurement by means of a capacitor.
2. Description of the Prior Art
The photometric circuit of a camera or the like which measures light in a wide brightness range generally produces the result of a light measurement by logarithmically compressing it for the purpose of broadening the photometric range thereof. For example, a popularly known photometric circuit of the logarithmic compression type is arranged as shown in FIG. 1 of the accompanying drawings. The conventional photometric circuit shown in FIG. 1 uses a transistor as the logarithmic compression element. The circuit comprises an operational amplifier 1; a photo-sensitive element 2 which receives light to be measured and photo-electrically transduces the light with two terminals thereof connected to input terminals of the operational amplifier 1; a transistor 3 which is of the PNP type has its emitter connected to the output terminal of the operational amplifier 1, its collector to the cathode of the photo-sensitive element 2 (an inversion input of the operational amplifier 1) and its base to the anode of the photo-sensitive element 2; a reference voltage source 4; and an output terminal 5 for producing the result of light measurement. Furthermore, the operational amplifier 1 is arranged to receives an energizing voltage +Vcc.
A current flowing from the photo-sensitive element 2 is proportional to the intensity of light incident on the element 2. A signal corresponding to the light is thus supplied to the operational amplifier 1. Since the transistor 3, which acts as the logarithmic compression element, is connected to the operational amplifier 1, the result of a light measurement produced from the output terminal 5 is of a logarithmically compressed value. Assuming that the voltage of the reference voltage source 4 is Er (Er&lt;&lt;Vcc), the output voltage V which is produced from the output terminal 5 can be expressed as follows: ##EQU1## wherein, K: Boltzmann's constant
q: elementary electric charge of electrons PA0 T: absolute temperature PA0 I: photo current flowing at the photo-sensitive element 2 PA0 Io: reverse biased saturation current
As shown in Formula (a) above, a signal corresponding to the brightness of the light received is logarithmically compressed, so that the result of a light measurement can be obtained covering a wide range of light brightness.
However, with the conventional photometric circuit arranged as described above, when the quantity of light incident on the photo-sensitive element 2 decreases, the photo current flowing at the photo-sensitive element 2 decreases to a great extent. In that event, a long period of time is required before an offset voltage, which exists at both terminals of the photo-sensitive element 2, and an excessive accumulated electric charge, which exists on the side of the collector of the transistor 3, come to disappear. Then, during that period, it is difficult to accurately carry out light measurement. In other words, charging and discharging processes for floating capacity and excess storage charge require a long period of time and the response speed of the photometric circuit decreases when the photo current becomes very small in low brightness.
Meanwhile, there have been proposed some methods for charging and discharging the floating capacity from outside. For example, Japanese Laid-Open Utility Model Application No. SHO 50-31931 discloses such a method. However, since a switching element is connected to a high impedance part of a logarithmic compression circuit in accordance with these methods, an error occurs since the floating capacity or a leakage current increases in such a case. Furthermore, in cases where an integration value of the light intensity must be obtained, such as with a flash device, the conventional photometric circuit of another type is arranged, as shown in FIG. 2, to have the photo current which flows at the photo-sensitive element 2 integrated directly by a capacitor 6; and the integrated result of light measurement is obtained from the output terminal 5, which is connected to the output terminal of the operational amplifier 1. However, in the event that the amplifier 1 which is used for amplifying the voltage of the capacitor 6 has an offset voltage, the photometric circuit has a photometric error. To solve this problem by reducing the offset voltage of the amplifier 1, an expensive amplifier of complex structural arrangement has been used. Furthermore, it is conceivable to use the same photo-sensitive element and the same operational amplifier for both the mode in which the result of light measurement is produced by logarithmically compressing it and for another mode in which the result of light measurement is produced by integrating it. An example of the conventional photometric circuits of this type is shown in FIG. 3. In this case, a change-over switch 7 is connected in series with the photo-sensitive element 2. The switch 7 shifts its connecting position to one side 7c connecting with a diode 9 for the logarithmic compression mode and the other side 7b connecting with the capacitor 6 for the integrating mode. This arrangement thus permits the use of the same photo-sensitive element 2 and operational amplifier 1 for the two modes. Furthermore, a switch 8 is connected parallel with the capacitor 6. When the switch 8 turned on, the capacitor 6 is reset with its two terminals shortcircuited.
In the case of the photometric circuit which is arranged as mentioned above, however, a leakage current occurs through the mode change-over switch 7 and the other switch 8 for resetting the capacitor 6 since they are connected to the inversion input terminal of the operational amplifier 1 which has an extremely high impedance. This leakage current and floating capacity bring about an error in the result of light measurement and thus necessitates a long period of time to obtain accurate light measurement.
The present invention is directed to the solution of the above-stated problem of the prior art. It is therefore an object of the invention to provide a photometric circuit which is capable of always correcting the offset voltages of a switching element and an amplifier, despite their magnitude and variations.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.